This invention relates to a method of evaluating the condition of a gapless metal-oxide varistor lightning arrester and, especially, though not exclusively, a lightning arrester of this type which is located within the housing of a distribution transformer. The invention also relates to apparatus for carrying out such a method.
In recent years, it has become increasingly common to locate the lightning arrester for a distribution transformer within the transformer tank, or housing, and thus under oil. This has been made possible by new designs of lightning arresters which employ metal-oxide varistors and have no gaps. Such arresters, which are referred to herein as gapless metal-oxide varistor type lightning arresters, are typically directly connected across the high voltage winding of the distribution transformer.
These arresters have typically been designed with a frangible end fitting so that in the rare event of an arrester failure, the failed arrester will be disconnected from the winding across which it is normally connected. The disconnection means is not designed as a fault-clearing device, but rather clearing is effected by a fuse or circuit breaker in the transformer supply circuit that opens in response to the fault.
Once an arrester has failed, the fault-clearing device has operated, and the arrester disconnection means has properly operated, the transformer can be reengerized for emergency use without arrester protection. It is possible that the arrester disconnection scheme may in some cases not properly operate and a faulted arrester will be left connected to the winding. In any case, when a lineman arrives to service the transformer it is desirable if he has a simple means of detecting whether the arrester is open, shorted, or normal and still connected to the winding before he energizes the transformer.